Organic Light emitting diode (OLED) and other organic electronic technologies such as organic photovoltaics (OPV) and organic field effect transistors are expected to be a major opportunity for advanced materials development impacting a large number of future technology based applications. These include flat panel displays which offer significant advantages over liquid crystal displays (LCDs) including much lower power requirements, improved definition, broader viewing angles, and faster response times. The technology for OLEDs offers the potential for lower cost lighting sources compared to incandescent lighting as well as fluorescent lighting applications. Inorganic based LEDs are already replacing some of these conventional applications including traffic lighting as well as flashlights offering equal or improved lighting at much lower power requirements.
Small molecule organic light emitting diodes (SMOLEDs) are being commercialized to replace LCD displays based on lower power requirements, faster response times, better definition, and also easier fabrication. Such SMOLEDs are expected to revolutionize the flat panel display technology. Another area receiving considerable interest involves polymeric light emitting diodes (POLEDs) where polymeric light emitting materials can be utilized for flexible organic light emitting diodes (FOLEDs). A significant advantage of polymeric materials involves the fabrication possibilities. FOLEDs offer the potential for ink-jet printing of flat panel displays on flexible substrates such as indium-tin oxide coated polymeric films (i.e. poly(ethylene terephthalate)(PET), oriented polypropylene or polycarbonate). Roll to roll printing processes could also be utilized for FOLEDs. The potential for FOLEDs is considered to be quite large offering unique flat or contoured display panels. These FOLEDs may be of interest for unique lighting applications and large screen displays. These displays would be low cost, easy to install, very thin, and power efficient. An example could be a battery operated TV screen, which would be the thickness of several sheets of paper and capable of folding, at a cost commensurate with the fabrication simplicity. Of course many problems have to be solved before these possibilities become reality.
Development of POLEDs has focused on polymeric materials which exhibit electroluminescence. These materials are generally conjugated polymers, such as poly (phenylene vinylene), polyfluorenes, polyphenylenes, polythiophenes, and combinations of such structures. Conjugated polymers for use in POLEDs are disclosed by a number of references including U.S. Pat. No. 5,247,190 to Friend et al., U.S. Pat. No. 5,900,327 to Pei et al. and Anderson et al., J. Mater Chem., 9, 1933-1940 (1999) which are hereby incorporated by reference in this specification in their entirely.
Variations of conjugated polymers useful for POLEDs include polymers comprised of oligomeric units of conjugated structures coupled into a high molecular weight polymer which are disclosed in U.S. Pat. No. 5,376,456 to Cumming et al., U.S. Pat. No. 5,609,970 to Kolb et al., Pinto et al., Polymer, 41, 2603-2611 (2000) and U.S. Pat. No. 6,030,550 to Angelopoulos et al and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
A large number of low molecular weight compounds are available which exhibit fluorescence and electroluminescence. Some of these materials are commonly referred to as laser dyes. Many of these compounds offer very high fluorescence and thus electroluminescence. However, the properties desired for OLED applications are generally only observed in solution or at low levels of doping in electro-optical or electroactive polymers. In the solid state, these materials can crystallize and lack the mechanical integrity to be utilized in POLEDs or SMOLEDs. Additionally (and more importantly), the excellent fluorescence and electroluminescence is lost with crystallization. These problems have been well documented in various reviews on the subjects of materials for OLEDs. See, e.g., Kelly, “Flat Panel Displays. Advanced Organic Materials.” (Royal Society of Chemistry, 2000) at pp. 155 and 177. Consequently, a number of attempts have been made to solve these problems.
For example, U.S. Pat. No. 6,329,082 to Kreuder et al. discloses hetero-spiro compounds suitable for use in OLED devices. The compounds purportedly overcome “the unsatisfactory film-forming properties and . . . pronounced tendency to crystallize” of conventional low molecular weight fluorescent materials.
U.S. Pat. No. 6,214,481 to Sakai et al. purports to address problems with low emission intensity in solution and thermal instability of OLEDs by providing an organic host compound (e.g., distyrylarylene derivatives) for a fluorescent substance, wherein the host compound has a fluorescent quantum efficiency of at least 0.3 in a solid state and a glass transition temperature (Tg) of at least 75 degrees Celsius (° C.).
Examples exist where fluorescent dopants are included in electroactive components of OLEDs. See, e.g., Shoustikov et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 1 (1998), Djurovich et al., Polymer Preprints, 41(1), 770 (2000), Chen et al., Polymer Preprints 41(1), 835 (2000), U.S. Pat. No. 6,303,239 to Arai, U.S. Pat. No. 4,769,292 to Tang et al., U.S. Pat. No. 6,329,086 to Shi et al., U.S. Pat. No. 5,928,802 to Shi et al., and Hu et al., J. Appl. Phys., 83(11) 6002 (1998).
Examples also exist in the literature where fluorescent dyes have been added to non-active polymers for various applications. See, e.g., Quaranta et al., Synthetic Metals, 124, 75-77 (2001), Muller et al., Polymer Preprints, 41(1), 810 (2000), Sisk et al., Chemical Innovation, May 2000, U.S. Pat. No. 6,067,186 to Dalton et al., Kocher et al., Advanced Functional Materials, 11(1), 31 (2001) and U.S. Pat. No. 5,952,778 to Haskal et al.
There are a number of examples in the literature where non-active polymers have been modified by side chain or main chain incorporation of optically active species. See, e.g., Hwang et al., Polymer, 41, 6581-6587 (2000), U.S. Pat. No. 5,414,069 to Cumming et al., U.S. Pat. No. 6,103,446 to Devlin et al., and United States patent application Publication United States 2001/0026879 A1 to Chen et al.
U.S. Pat. No. 6,277,504 to Koch et al. discusses an electroluminescent assembly comprising a component which is a substituted or Unsubstituted 1,3,5-tris (aminophenyl) benzene and a luminescent compound based on substituted metal complexed hydroxyquinoline compounds. The electroluminescent assembly can further comprise a polymeric binder. Similarly, U.S. Pat. No. 6,294,273 to Heuer et al. discloses a polymeric binder for the electroluminescent compound of a metal complex of N-alkyl-2,2′-imino-bis (8-hydroxy-quinoline).
Various references note blends of active electroluminescent polymers for utility in OLED devices offering in many cases improved performance over the individual constituents. See, e.g., Hu et al., J. Appl. Phys., 76(4), 2419 (1994), and Yang et al., Macromol. Symp., 124, 83-87 (1997).
Blends of fluorene-based alternating polymer with non-active polymers (e.g. PMMA, epoxy resin, polystyrene) are disclosed in U.S. Pat. No. 5,876,864 to Kim et al. U.S. Pat. No. 6,255,449 to Woo et al. notes the utility of blends of specific fluorene containing polymers and a litany of other polymers, including conjugated polymers.
Frederiksen et al., J. Mater. Chem., 4(5), 675-678 (1994) teaches the addition of laser dyes to a polystyrene matrix for use in an OLED device.
U.S. Pat. No. 5,821,003 to Uemura et al. notes the use of polymeric binders for low molecular weight hole transport materials for the hole transport layer of OLED devices. Examples include polysulfone and aromatic tertiary amines. The inclusion of minor amounts of fluorescent compounds in the polymer bound hole transport layer is noted to improve the luminance of blue and white.
U.S. Pat. No. 5,663,573 discloses the use of a variety of organic light emitting materials for preparing a bipolar electroluminescent device, including polypyridines, polypyridylvinylenes, polythiophenes, polyphenylenes, polyphenylenevinylenes, polyphenylenebenzobisthiazoles, polybenzimidazobenzophenanthrolines, polyfluorenes, polyvinylcarbazoles, polynaphthalenevinylenes, polythienylenevinylenes, polyphenyleneacetylenes, polyphenylenediacetylenes and polycyanoterephthalylidenes.
U.S. Pat. Nos. 6,818,919 and 7,115,430 attempt to address this problem by providing a light emitting device comprising:
a light emitting layer comprising an electroluminescent organic material dispersed in a matrix, wherein the electroluminescent organic material has a molecular weight less than about 2000 amu, the matrix comprises a non-electroluminescent organic polymer having a Tg of at least 170° C., and each of the non-electroluminescent organic polymer and the electroluminescent organic material constitutes at least 20 percent by weight of the light emitting layer; and
electrodes in electrical communication with the light emitting layer and configured to conduct an electric charge through the light emitting layer such that the light emitting layer emits light.
Further provided is a method for manufacturing a light emitting device, comprising providing the light emitting layer; and providing electrodes in electrical communication with the light emitting layer, wherein the electrodes are configured to conduct an electric charge through the light emitting layer such that the light emitting layer emits light. U.S. Pat. Nos. 6,818,919 and 7,115,430 disclose that in the case of laser dyes one class of low molecular weight electroluminescent materials, the rate of crystallization of the laser dyes ((which characteristically exhibit crystallinity) is dependent upon the Tg of the polymer/electroluminescent material mixture. If the T9 of the polymer/laser dye is exceed, crystallization of the laser dye could occur thus limiting the electroluminescent efficiency of the device. Thus U.S. Pat. Nos. 6,818,919 and 7,115,430 to Robeson et al, claim that high Tg polymers can be utilized to prevent the crystallization of laser dyes even when the laser dye concentration is in excess of 50 wt. % based on the weight of the light emitting film. However, example 20 of U.S. Pat. Nos. 6,818,919 and 7,115,430 (polysulfone/Coumarin 6, 2/1 blend) clearly shows crystallization of the Coumarin 6 on the first and second heating of the differential scanning calorimetry (DSC) results, even in the presence of residual tetrahydrofuran (THF) coating solvent. These results suggest that the amorphous mixture below the T9 of the blend is a “kinetic glass”. It is well known by those versed in the art that such glass will eventually crystallize thermodynamically, resulting in performance degradation of the device.
All references cited herein are incorporated herein by reference in their entireties.
There is still a need for a light emitting layer, incorporating the excellent properties of low molecular weight electroluminescent materials such as laser dyes, that is truly noncrystallizable, for use in SMOLED devices, POLED devices, combination SMOLED POLED devices, and FOLED devices.
There is further need for a noncrystallizable light emitting layer that can be coated by spin coating, roll to roll coating, or inkjet and other deposition methods, without much restriction in the coating conditions required to provide solvent-free film required for long-lived and stable devices.
The present invention provides a solution for the above problems.
It is an object of this invention to provide light emitting layers with the many of the advantages illustrated herein.
It is also an object of this invention to provide truly noncrystallizable light emitting layers, containing both low molecular-weight electroluminescent materials such as laser dyes, and low molecular-weight charge transport materials.
It is yet another object of this invention to provide truly noncrystallizable light emitting layers that can be easily coated by fabrication technique selected from the group consisting of spin coating, screen printing, ink jet printing and roll-to-roll printing
It is also an object of this invention to provide noncrystallizable sensitized layers for other organic electronic applications such as OPV, OTFT, photochromic devices based on nonpolymeric amorphous glass mixture compositions.